The consistent production of foamed thermoplastics materials (for example foamed polyethersulphone--hereinafter referred to as P.E.S.--polysulphone, polycarbonates and the like aromatic polymers) present several problems. One great problem is the production of a suitable preliminary material, precursor or pre-form for the foaming process.
In general a foamed thermoplastics material is formed by heating a pre-form or precursor material leading to a substantial expansion in its volume, and a substantial decrease in its density. To produce consistently high quality foamed thermoplastics material it is necessary that the pre-form material contains no large, visible, random, irregular holes which would lead to irregularities in the size and structure of the cells of the finished, foamed material. Such holes are hereafter referred to as `voids`.
The known techniques for producing pre-form materials essentially comprise soaking granules of a thermoplastics material to be foamed in a solvent for a considerable period of time (say 170 to 350 hours) and then heating the resultant gel to produce a foam of expanded material with a density substantially less than that of the original thermoplastics material. For example when foaming P.E.S. having an original density of approximately 1370 kgs/m.sup.3, steeping granules of P.E.S. in acetone and heating produces an expanded, foamed material with a density of approximately 100 kgs/m.sup.3.
The main problem with this method is that air becomes trapped in the gel pre-form material and causes irregularities in the size and structure of the cells in the finished product; the size of holes produced is usually so large that the material is of little use in applications requiring a uniform and readily reproducible product.
It is known that if the gel pre-form is allowed to dry for a considerable period of time (two to three months) after steeping, it is possible to produce a finished article having smaller and more regular cells. However large irregular `voids` still occur in such materials.